Amalgam for use in fluorescent lamps comprising lead, tin, mercury together with another of the group silver, magnesium, copper, nickel, gold and platinum.

ABSTRACT

An amalgam for a fluorescent lamp. The amalgam includes mercury, tin, lead, and another metal selected from the group consisting of silver, magnesium, copper, gold, platinum, and nickel.

BACKGROUND OF THE INVENTION

The present invention relates to amalgams for use in fluorescent lampsand other devices which require amalgams.

As discussed in "Amalgams for Fluorescent Lamps" by J. Bloem et alPhilips Technical Review 38, P 3-88 1978/79 No. 3, the luminous flux ofa fluorescent lamp (low-pressure mercury type) depends to a considerableextent on the mercury-vapour pressure present in the tube. The pressureis determined by the temperature of the coolest part of the tube, whichis usually the wall. The maximum luminous flux is reached when the walltemperature is 40° C. which for many fluorescent lamps corresponds to anambient temperature of 25° C. The wall temperature of lamps in closedluminaires or special lamps can be very much higher. In such conditionsa high luminous flux can still be attained by using a suitable amalgamin place of pure mercury. This has the effect of lowering the mercurypressure and also of keeping it more or less stable over a broadtemperature range. Bloem et al disclose various amalgams containingmercury and one or more of the metals Pb, Sn, Bi, In, Cd, Ga and Ti andstate that of amalgams with three or four metals Bi--Pb--Sn--Hg seemedto be the most promising. Bloem et al discuss Bi₀.53 In₀.47 with 6atomic percent Hg and Bi₀.47 Pb₀.29 Sn₀.24 with 6 atomic percent Hg,which gave the best results. These amalgams give good stable luminousflux at higher than ambient temperature without the mercury-vapourpressure being too low at ambient temperature thus allowing the lamp toreach its maximum luminous flux.

For a better understanding of the prior art and of the inventionreference will be made in the following discussion to the accompanyingdrawings which shows pressure temperature curves of various amalgams.

U.S. Pat. No. 4,924,142 discloses an amalgam comprising Hg, In, Sn andZn, wherein the ratio between the atoms of In and Sn is between 3:1 and8:1; the ratio between the sum of the atoms of In and Sn and the atomsof Zn is between 95:5 and 99:1; and the ratio between the sum of theatoms of In, Sn and Zn and the atoms of Hg is between 95:5 and 99:1.U.S. Pat. No. 4,924,142 discloses an example of the amalgam in which theatomic ratio of the elements In: Sn: Zn is 82.5: 16:1.5, with 2 atomicpercent Hg. The temperature pressure curve of such an amalgam is shownby curve 4924142 in the accompanying drawing. The curve shows that thetemperature interval of the operating range of the amalgam (the plateau)is between 105° C. and 130° C. where the plateau pressure is about3×10⁻³ Torr.

U.S. Pat. No. 4,615,846 discloses an amalgam consisting of 15 to 57 wt %Sn, 5 to 40 wt % Pb, 30 to 70 wt % Bi, 4 to 50 wt % In and 4 to 25 wt %Hg. The temperature pressure curve of such an amalgam is shown by curve4615846 in the accompanying drawing. The curve shows that within thetemperature range 50° to 130° C. the mercury vapour pressure is held at6×10⁻³ to 7×10⁻³ mm Hg (Torr).

EP-B1-0,157,440 discloses an amalgam of Hg and an alloy wherein thealloy is composed of bismuth, lead and silver whereby the mutual ratioof the numbers of atoms of bismuth, lead and silver lies in thequadrangle ABCD of the ternary diagram Bi--Pb--Ag with

A:93% of Bi, 2% of Pb, 5 of Ag;

B:35% of Bi, 60% of Pb, 5% of Ag;

C:35% of Bi, 35% of Pb, 30% of Ag;

D:68% of Bi, 2% of Pb, 30% of Ag (atomic %),

and that the ratio of the sum of the number of atoms of bismuth, leadand silver to the number of atoms of mercury lies between 94:6 and 99:1.

Curve 0157440 on the accompanying drawing shows the pressure temperaturecurve of such an amalgam where the atom ratio of Bi:Pb:Ag:Hg=53:24:20:3.At about 110° C. the mercury vapour pressure is about 1.5 Pa (11×10⁻³ mmHg).

This allows good light output at high temperature, the pressure of 1.5Pa being maintainable over a range of temperatures as indicated by theplateau about 110° C.

SUMMARY OF THE INVENTION

In accordance with the present invention, an amalgam is providedincluding 0.001 to 0.005 mole fraction of mercury, 0.24 to 0.48 molefraction of lead, 0.45 to 0.73 mole fraction of tin, and 0.01 to 0.15mole fraction of other metal selected from the group consisting ofsilver, magnesium, copper, gold, platinum, and nickel. Also provided inaccordance with the present invention is a fluorescent lamp thatincludes the foregoing amalgam.

BRIEF DESCRIPTION OF THE DRAWING

The features, aspects, and advantages of the present invention willbecome better understood with regard to the following description,appended claims, and accompanying drawing, which shows a graph oftemperature versus pressure for several prior art amalgams, as well asthe inventive amalgam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, there is provided the followingamalgams:

Silver+Lead+Tin+Mercury

Magnesium+Lead+Tin+Mercury

Copper+Lead+Tin+Mercury

Gold+Lead+Tin+Mercury

Platinum+Lead+Tin+Mercury

Nickel+Lead+Tin+Mercury

It is expected that any metal which is added to the system(Lead+Tin+Mercury) which shows:

Intermetallic compound formation with Tin and

Binary eutectic formation at high Lead contents would have a region ofstability as described below and therefore be suitable for use as anamalgam system for use in fluorescent lamps.

It has been found that these amalgam systems work above 130° C.

Furthermore, a surprising property of such systems, not hithertoexpected, is a region of slowly changing vapour pressure withtemperature which can be utilised to give optimum mercury vapourpressure for a fluorescent lamp at a high temperature.

Over a restricted composition range of the parent alloy, e.g.Silver+Lead+Tin, amalgams with low mercury content (typically 0.2 molepercent) exhibit a small change in mercury partial pressure over a largetemperature range. In the currently preferred example below the partialpressure of mercury varies by approximately 1.4 Pa over the range 180°to 280° C. This characteristic renders these amalgam systems extremelyattractive for fluorescent lamps with high cool spot temperatures.

The systems according to the invention and described above differ fromthose commonly used and those reported in the literature in that thereis a greater temperature range over which vapour pressure changes slowlywith temperature. In the inventive systems above, this temperature rangecovers approximately 100° C. and in commonly used systems it coversapproximately 40° to 70° C. The latter systems are also restricted to amaximum temperature of 150° C.

Preferred amalgams in accordance with the invention include those havingthe following composition in mole fractions:

    ______________________________________           mercury     0.001 to 0.005           lead        0.24 to 0.48           tin         0.45 to 0.73    ______________________________________

and the other metal 0.01 to 0.15, the other metal comprising silver,magnesium, copper, gold, platinum or nickel.

A presently preferred embodiment of the invention comprises Pb₀.47Sn₀.47 Ag₀.06 with 0.2 atomic percent Hg where the proportions of theelements are in mole fractions. Such an amalgam has the characteristicof partial mercury pressure versus temperature as shown by the curvedenoted INVENTION in the drawing. This characteristic is surprising andnot hitherto expected. It has a region A in which the mercury vapourpressure varies by a relatively small amount e.g. about 3×10⁻³ Torrbetween about 180° C. and 230° C., i.e. over a temperature range ofabout 50° C. The mercury vapour pressure also varies by a similar amountover the next temperature range of about 50° C. (i.e. up to about 280°C.), thus providing a wide temperature range (about 100° C.) with only arelatively small change in vapour pressure.

The temperatures in the region A of small change are considerably higherthan the corresponding regions of the prior art.

The relatively elevated temperatures of the range A of slow changeallows greater freedom in the design of fluorescent lamps, especiallyElectrodeless Fluorescent Lamps.

Other amalgams according to the invention include:

Pb₀.48 Sn₀.48 Ag₀.04 with 0.1 atomic percent Hg

Pb₀.24 Sn₀.74 Ag₀.02 with 0.1 atomic percent Hg

all of which have a pressure/temperature characteristic like (but notidentical to) that shown in the drawings by the curve INVENTION.

What is claimed is:
 1. An amalgam comprising:

    ______________________________________    mercury         0.001 to 0.005 mole fraction    lead             0.24 to 0.48 mole fraction    tin              0.45 to 0.73 mole fraction    other metal      0.01 to 0.15 mole fraction    ______________________________________

wherein said other metal is selected from the group consisting ofsilver, magnesium, copper, gold, platinum, and nickel.
 2. An amalgamcomprising, in mole fractions Pb₀.47 Sn₀.47 Ag₀.06 with 0.2 atomicpercent Hg.
 3. An amalgam comprising, in mole fractions Pb₀.48 Sn₀.48Ag₀.04 with 0.1 atomic percent Hg.
 4. An amalgam comprising in molefractions Pb₀.24 Sn₀.74 Ag₀.02 with 0.1 atomic percent Hg.
 5. Afluorescent lamp including an amalgam, said amalgam comprising:

    ______________________________________    mercury         0.001 to 0.005 mole fraction    lead             0.24 to 0.48 mole fraction    tin              0.45 to 0.73 mole fraction    other metal      0.01 to 0.15 mole fraction    ______________________________________

wherein said other metal is selected from the group consisting ofsilver, magnesium, copper, gold, platinum and nickel.
 6. A lampaccording to claim 5 which is an electrodeless fluorescent lamp.